This invention relates in general to a catalyst and process for the isomerization of alkyl aromatics. In particular, the invention relates to improvements in the commercial processes in which a mixture of xylenes and ethylbenzene are reacted along with hydrogen over a platinum-containing catalyst to produce a near equilibrium mixture of xylenes while converting ethylbenzene to xylenes.
Isomerization of alkyl aromatics has become particularly important commercially. In processes for the production of xylenes, the ortho and paraxylenes are the preferred products. Paraxylene is principally used in preparation of polyesters, while orthoxylene's main end use is in preparation of phthalic anhydride. Metaxylene has fewer important end uses and thus may be converted to the para and ortho forms, which have greater commercial value.
Ethylbenzene is difficult to separate from the xylenes since their boiling points are very close. Accordingly, ethylbenzene is normally present within the mixture of xylenes prepared by extraction or distillation from a hydrocarbon stream. Generally there are two approaches to the problem of processing the ethylbenzene in a mixed xylene stream. Because ethylbenzene is not easily isomerized one approach has been to destroy the ethylbenzene through disproportionation, hydrodealkylation or the like to yield lighter and heavier compounds which can be separated by distillation from the C.sub.8 compounds. One drawback to this approach has been the accompanying loss of significant quantities of potential xylenes in this reaction.
It is also known in the prior art to react ethylbenzene to form xylenes in the presence of hydrogen and a hydrogenation-dehydrogenation catalyst, preferably platinum on alumina. U.S. Pat. No. 2,976,332 discloses a catalyst comprising platinum on alumina plus an amorphous silica/alumina to convert ethylbenzene and to isomerize xylenes with a minimum of side reactions which reduce the yield selectivity and contribute to catalyst aging.
Combination catalysts which are capable of isomerizing xylenes and ethylbenzenes to approach an equilibrium distribution of isomers include U.S. Pat. No. 2,976,332 which combines platinum on alumina with amorphous silica-alumina. Another combination catalyst is disclosed in U.S. Pat. No. 3,409,686 in which alumina gel is mixed with particles of hydrogen mordenite to form a mixed base which is subsequently dried and impregnated with a platinum solution. In U.S. Pat. No. 3,767,721 platinum on alumina in a fine powdered form is combined with powdered mordenite. Hydrogen mordenite in such a combination, however, is overly active and promotes destructive reactions which are undesirable. U.S. Pat. No. 4,128,591 discloses a catalyst and process for producing a near equilibrium mixture of xylene from a feedstream comprising ethylbenzene and mixed xylenes. The catalyst combines a platinum containing hydrogenation/dehydrogenation component on an alumina support with a form of hydrogen mordenite.
The prior art discloses a number of catalysts for isomerization of xylenes alone or combined with the conversion of ethylbenzenes to xylenes. While some of the prior art catalysts have been successful in isomerizing xylenes in ethylbenzene, further improvement has been desired in order to achieve a more selective catalyst which can be operated to approach chemical equilibrium in the isomerization of xylenes and ethylbenzene while at the same time avoiding the destructive reactions which can result in a loss of C.sub.8 aromatics.